As electrical circuits become optimized, margins become increasingly stricter. These are margins of variations in both the mass manufacturing of the devices (to ensure uniformity), and also in their operation.
One type of margin that is affected is variations in the actual values of reference voltages. These are voltages that the circuit treats as having a known and substantially constant value. These reference voltages can be affected by variations in operating temperature of the circuit. They can also be affected by variations in manufacturing, since individual components may be manufactured with values different than designed.